Due to the increasing demand for wireless communication, it has become necessary to develop techniques for more efficiently using the allocated frequency bands, i.e., increasing the capacity to communicate information within a limited available bandwidth. In conventional low-capacity wireless communication systems, information is transmitted from a base station to subscribers by broadcasting omnidirectional signals on one of several predetermined frequency channels. Similarly, the subscribers transmit information back to the base station by broadcasting similar signals on one of the frequency channels. In this system, multiple users independently access the system through the division of the frequency band into distinct subband frequency channels. This technique is known as frequency division multiple access (FDMA).
A standard technique used by commercial wireless phone systems to increase capacity is to divide the service region into spatial cells. Instead of using just one base station to serve all users in the region, a collection of base stations are used to independently service separate spatial cells. In such a cellular system, multiple users can reuse the same frequency channel without interfering with each other, provided they access the system from separated spatial cells. The cellular concept, therefore, is a simple type of spatial division multiple access (SDMA). In order to allow the subscriber units to identify and communicate with separate cells, the base station for each cell continually broadcasts an omni-directional control signal to all users in the cell. This signal is traditionally transmitted on a special control channel and can contain various types of information for signal synchronization, control, etc.
In the case of digital communication, additional techniques can be used to increase capacity. A few well-known examples are time division multiple access (TDMA) and code division multiple access (CDMA). TDMA allows several users to share a single frequency channel by assigning their data to distinct time slots. CDMA is normally a spread-spectrum technique that does not limit individual signals to narrow frequency channels but spreads them throughout the frequency spectrum of the entire band. Signals sharing the band are distinguished by assigning them different orthogonal digital code sequences. CDMA is generally considered the most promising multiple access technique in the cellular telephone industry.
Despite the promise of CDMA, practical issues such as power control speed and inter-base station interference have limited system effectiveness in its initial phase of implementation. CDMA-based system capacity depends very much on the ability to provide for very accurate power control; but in a mobile environment, the signal may fluctuate too fast for the system to manage effective control. In addition, mobile wireless environments are often characterized by unstable signal propagation, severe signal attenuation between the communicating entities and co-channel interference by other radio sources. Moreover, many urban environments contain a significant number of reflectors (such as buildings), causing a signal to follow multiple paths from the transmitter to the receiver. Because the separate parts of such a multipath signal can arrive with different phases that destructively interfere, multipath can result in unpredictable signal fading. In addition, in order to provide service to shadowed areas, radiated power is increased, thereby increasing interference between base stations and significantly degrading overall system performance.
Recently, considerable attention has focused on ways to increase wireless system performance by further exploiting the spatial domain. It is well-recognized that SDMA techniques could, in principle, significantly improve the performance of a CDMA-based network. In practice, however, such significant improvements have yet to be realized. Current approaches are either simple but not very effective or effective but too complex for practical implementation.
One well-known SDMA technique provides the base station with a set of independently controlled directional antennas, thereby dividing the cell into separate fixed sectors, each controlled by a separate antenna. In order to allow the subscriber units to distinguish separate sectors in a cell, the base station continually broadcasts in each sector a fixed directional control signal that is unique to the sector. This technique allows subscribers in separate spatial sectors of the same cell to be spatially distinguished by the base station. As a result, the frequency reuse in the system can be increased and/or cochannel interference can be reduced. A similar but more complex technique can be implemented using a coherently controlled antenna array instead of independently controlled directional antennas to form fixed sectored beams. Using a signal processor to control the relative phases of the signals applied to the antenna array elements, predetermined downlink beams can be formed in the directions of the separate sectors. Similar signal processing can be used to selectively receive uplink signals only from within the distinct sectors. These sectoring techniques, however, only provide a relatively small increase in capacity compared to what is theoretically possible.
More sophisticated SDMA techniques have been proposed that theoretically could dramatically increase system capacity. For example, Gerlach et al. (U.S. Pat. No. 5,471,647 and U.S. Pat. No. 5,634,199) and Barratt et al. (U.S. Pat. No. 5,592,490) disclose wireless communication systems that increase performance by exploiting the spatial domain. In the downlink, the base station determines the spatial channel of each subscriber and uses this channel information to adaptively control its antenna array to form customized narrow beams. These beams transmit an information signal over multiple paths so that the signal arrives to the subscriber with maximum strength. The beams can also be selected to direct nulls to other subscribers so that cochannel interference is reduced. In the uplink, the base station uses the channel information to spatially filter the received signals so that the uplink signal is received with maximum sensitivity and distinguished from the signals transmitted by other subscribers. Through selective power delivery by intelligent directional beams, the interference between base stations can be reduced and the carrier-to-interference ratio at the base station receivers can be increased.
One of the most significant problems with these adaptive beamforming techniques is the computational complexity required to estimate the wireless air channel. In a typical base station that must simultaneously determine beams in real time for more than 100 subscriber units, the computational power required to implement the known techniques is presently beyond practical realization. Another problem with adaptive beam forming methods described in the art (e.g., U.S. Pat. No. 5,434,578) is that they deal only with uplink estimation. Downlink channel estimation, however, is a much more difficult problem. In particular, because the spatial channel is frequency dependent and the uplink and downlink frequencies are usually different, the uplink beamforming techniques do not provide the base station with sufficient information to accurately derive the downlink spatial channel information and improve system capacity.
For example, Borras et al. (U.S. Pat. No. 5,303,240) discloses a technique wherein a subscriber transmits a training signal in the uplink to a base station with an antenna array during a training mode. After the antenna array determines the beams corresponding to the best uplink reception during this training mode, the array then uses these beams for uplink and downlink beamforming. This technique, however, does not accurately measure correct downlink spatial beam information because the spatial channel is frequency dependent and the uplink and downlink frequencies are often different. The technique of Borras also suffers from the disadvantage that a separate training mode is required to determine the spatial channel. This approach has the disadvantage that the information transmission mode must be interrupted frequently to update the beam information whenever the propagation environment is not stable.
A technique for obtaining actual downlink channel information is to transmit probing signals and receive feedback from the subscriber, as taught by Gerlach et al. Gerlach's probing signals are omni-directional signals transmitted separately from the individual antennas in the array. The subscriber then detects the probing signals and determines the downlink spatial channel for each antenna. This downlink channel information is then sent back to the base station by the subscriber. In principle, the base station can then use the downlink channel information when transmitting downlink signals, thereby improving system performance. In practice, however, Gelach's technique has the significant problem that in typical environments where the propagation environment is changing, it requires very high feedback rates to transmit channel information for all the antennas.
There is a need, therefore, for downlink beamforming methods that overcome the limitations in the known approaches.